Embodiments of the present invention relate generally to calibration of systems for inspecting electronic assemblies such as, but not limited to, circuit cards and printed circuit boards (PCBs).
An X-ray system may be used to inspect such assemblies. For example, a PCB on which a ball grid array (BGA) is present may be inspected for defects. Such defects may include portions of the BGA where solder did not reflow properly during manufacturing and/or where the solder reflow process was similarly deficient or otherwise insufficient. As a result, defects such as bridges and ball-and-socket-type opens may be present.
One metric used to measure performance of an inspection system or process is the so-called “false calls” rate. This metric reflects the number of times an acceptable (or non-defective) joint of an assembly is incorrectly or unnecessarily identified as being unacceptable (or defective) during inspection. For example, in one scenario, an acceptable joint is incorrectly identified as being unacceptable by one system. However, the same joint is correctly identified as being acceptable by another system. The incorrect identification by the first system is an example of an occurrence that would result in an increase of the false calls rate. Ideally, the first system should have identified the acceptable joint as being acceptable, thereby preempting an increase of the false calls rate. To improve efficiency, the accuracy of inspection systems should be improved such that the false calls rate is decreased.
Calibration of inspection systems, such as the system described above, may be performed to provide various features. For example, as described above, such systems may be calibrated to improve stability and/or consistency across results provided by different systems. In addition, calibration may be used to improve stability, reproducibility and/or consistency across results provided at different times by a same system (for example, inspections performed over the operational lifetime(s) of one or more parts or portions of the system). Furthermore, calibration may be performed such that adverse effects of so-called “systematic errors” (i.e., repeatable errors that can be removed through calibration) are effectively removed or dampened. Such systematic errors may be caused, for example, by geometrical deficiencies, defects (or other imperfections) in system design, performance characteristics of system parts (e.g., the X-ray source and/or detection sensors), and other non-ideal operating conditions. Those skilled in the art will appreciate that the performance of an X-ray system may be negatively affected by both systematic errors and random noise.